The present invention relates to an imaging lens for forming an image of an object on an imaging element such as a CCD sensor and a CMOS sensor. In particular, the present invention relates to an imaging lens suitable for mounting in a relatively small camera, such as a camera for mounting in a portable device, e.g., a cellular phone, a smartphone, a portable information terminal, and a digital still camera, a security camera, a vehicle onboard camera, and a network camera.
In these years, in place of cellular phones that are intended mainly for making phone calls, so-called “smartphones” have been more widely used, i.e., cellular phones with such functions as portable information terminals (PDA) and/or personal computers. Since the smartphones generally are highly functional as opposed to the cellular phones, it is possible to directly process images taken by a camera thereof in various applications without importing in personal computers or the like. In case of the smartphones, it is possible to use the images taken with the cameras thereof in various applications. For example, it is possible to use the images for game characters of online games or for models in makeup simulations. Such uses of the images, which were not conventionally common, are becoming increasingly popular every year.
Generally speaking, an imaging lens for mounting in a portable device such as a cellular phone or a smartphone, which is developed for advanced users, requires optical performances comparable to digital still cameras, including high resolution and color reproducibility, as well as a small size due to limited space in a portable device. On the other hand, in case of uses in the above-described applications, it is not always necessary that an imaging lens for mounting in a camera has high resolution, but rather, it is more important that an imaging lens has a wide angle of view, i.e., a wide angle, for some applications.
A lens configuration composed of three lenses can relatively satisfactorily correct aberrations and is also suitable for downsizing, so that it has been widely used as a lens configuration for the above-described imaging lens of a camera for mounting in a portable device. When the imaging lens has the three-lens configuration, one of most well-known configurations includes a first lens having positive refractive power, a second lens having negative refractive power, and a third lens having positive or negative refractive power, arranged in the order from an object side.
According to the lens configuration described above, the second lens has negative refractive power. Accordingly, it is relatively easy to achieve downsizing of the imaging lens. In addition, since it is also possible to correct a chromatic aberration generated in the first lens through the second lens, it is one of effective lens configurations in view of downsizing of an imaging lens and satisfactory correction of aberrations.
According to the lens configuration described above, however, the second lens has negative refractive power. Accordingly, when it is attempted to achieve both a small size of an imaging lens and a wide angle, refractive power of each lens tends to be relatively strong. Further, it is often necessary to increase fabrication accuracy and/or assembling accuracy of the lens. For this reason, recently, there has been also a proposal of a lens configuration, in which refractive powers of all the three lenses are positive. As an imaging lens having such three-lens configuration, for example, there is known the one described in Patent Reference.    Patent Reference: Japanese Patent Application Publication No. 2012-14139
According to the imaging lens disclosed in Patent Reference, the second lens and the third lens are formed to have aspheric surfaces having inflexion points. When the lenses have the aspheric surfaces, it is achievable to obtain a small size of the imaging lens and a wide angle, as well as satisfactorily correcting aberrations.
According to the imaging lens described in Patent Reference, it is achievable to downsize the imaging lens while satisfactorily correcting aberrations to some extent. However, uses of images taken with a camera have been more and more various each year, and even wider angle is demanded as one of added values of the imaging lens. Although the imaging lens described in Patent Reference has a wider angle than conventional one, there is a limit itself in achieving even wider angle.
Here, it is not a problem specific to an imaging lens for mounting in a portable device such as cellular phone or smartphone to attain both a small size and a wide angle while satisfactorily correcting aberrations. Such a problem is common also among imaging lenses for mounting in relatively small cameras such as digital still cameras, security cameras, vehicle onboard cameras, and network cameras.
In view of the above-described problems in conventional techniques, an object of the present invention is to provide an imaging lens that can attain both a small size and a wide angle, while satisfactorily correcting aberrations.
Further objects and advantages of the invention will be apparent from the following description of the invention.